This invention relates to electron beam lithographies and particularly to an electron beam apparatus suitable for uniformly exposing the area to be exposed, by an electron beam.
In a conventional electron beam lithography, when the electron beam is blanked, a voltage is applied to electrostatic electrodes provided at a position along the electron beam path in synchronism with a lithographing signal, to thereby deflect the electron beam out of the opening of an aperture plate provided in the electron beam path. In addition, when the electron beam is irradiated on a sample, thereby exposing it, the voltage to the electrostatic electrodes is removed in synchronism with the lithographing signal so that the electron beam is released from the blanked state and passed through the opening of the aperture plate to the sample for a predetermined time. Thereafter, the electron beam is again blanked in synchronism with the image signal, thereby making a desired exposure on the sample.
The amount of electron beam irradiated on the sample with respect to time will be described with reference to FIGS. 5(a), 5(b) and 5(c).
When exposure is made at a predetermined point c on the sample by a beam spot of 4 .mu.m.times.4 .mu.m size, the electron beam is moved as indicated by A, B and C (see FIG. 5(b)) in synchronism with a blanking voltage signal V (see FIG. 5(a)), stops at point c for time t.sub.b and again is moved as indicated by C, B and A, completing the exposure. FIG. 5(b) shows the situation in which the beam is moved on the sample. At this time, the amounts of charge, Q.sub.a and Q.sub.b irradiated at points a and b can be expressed as follows: EQU Q.sub.a =J.multidot.(t.sub.b +2t.sub.a) (1) EQU Q.sub.b =J.multidot.t.sub.b ( 2)
where J is the current value.
Therefore, the difference between the amounts of charge irradiated on points a and b is expressed as EQU .DELTA.Q=Q.sub.a -Q.sub.b =2Jt.sub.a ( 3)
If the amount of charge at point a is assumed to be the optimum exposure condition, the amount of charge at point b is .DELTA.Q short for exposure. Thus, when it is developed through a process, the size at point c is smaller than 4 .mu.m. On the other hand, when the sensitivity of the photoresist is increased, the optimum exposure condition Q.sub.a decreases and as a result the rate of .DELTA.Q to Q.sub.a is large. Thus, insufficient exposure will greatly increase the dimensional error.